Application of NIR for disease assessment

Abstract

A problem with determining levels of powdery mildew in the vineyard or at the weighbridge is that visual assessment is subjective, varying between assessors and often being over-estimated. As the assessment of downy mildew infection affects returns to grape growers and also has implications for winemaking, more objective measures were assessed. These included DNA ("genetic fingerprinting") and spectroscopy.

Summary

A major contribution to the study was made by the Hardy Wine Company, whose staff collected diseased samples of grapes in the 2006 vintage from vineyards in the Riverland and McLaren Vale. Samples were visually classified for severity of powdery mildew using a key based on the percentage of the surface area of bunches affected.  Bunches were then frozen and taken to a laboratory at the Waite Campus where samples were homogenised and subjected to tests using advanced analytical equipment to provide an accurate account of actual infection. This could then be compared with visual assessments and measurements made using DNA extractions, near infra red scanning and other means.

The relationship between disease severity, content of powdery mildew DNA and NIR spectra on samples from contrasting viticultural regions was, in general, confirmed. It was concluded that NIR could provide an objective method for rapid assessment of powdery mildew contamination at the weighbridge, as demonstrated by previous work. This method, however, is correlative and requires a reference point for calibration and validation. The study showed DNA could be this reference point. Meanwhile, although visual assessment seems to be an unreliable reference point at the winery, it was suggested that accuracy for vineyard assessment purposes could be improved with training.